Leakage efficient anti-glitch filter with variable delay stages

ABSTRACT

A leakage efficient anti-glitch filter with variable delay stages. In accordance with a first embodiment of the present invention, a leakage efficient anti-glitch filter with variable delay stages comprises a plurality of variable delay stages and a coincidence detector element for detecting coincidence of an input signal to the delay element and an output of the delay element. The plurality of variable delay stages may comprise stacked inverter circuits or stacked NAND circuits.

RELATED APPLICATIONS

This application is a Continuation-in-Part of co-pending, commonly owned U.S. patent application Ser. No. 10/864,271, filed Jun. 8, 2004, entitled “Stacked Inverter Delay Chain” to Masleid and Burr, which is hereby incorporated herein by reference in its entirety.

This application is related to the following co-pending, commonly assigned United States patent applications, which are hereby incorporated herein by reference in their entirety:

U.S. patent application Ser. No. 11/021,222, filed Dec. 23, 2004, entitled “A Configurable Tapered Delay Chain with Multiple Sizes of Delay Elements” by Masleid;

U.S. patent application Ser. No. 11/021,221, filed Dec. 23, 2004, entitled “A Configurable Delay Chain with Switching Control for Tail Delay Elements” by Masleid;

U.S. patent application Ser. No. 11/021,632, filed Dec. 23, 2004, entitled “Power Efficient Multiplexer” to Masleid;

U.S. patent application Ser. No. 11/020,746, filed Dec. 23, 2004, entitled “A Configurable Delay Chain with Stacked Inverter Delay Elements” by Masleid; and

U.S. patent application Ser. No. 11/021,633, filed Dec. 23, 2004, entitled “Leakage Efficient Anti-glitch Filter” by Masleid.

FIELD OF THE INVENTION

Embodiments in accordance with the present invention relate to leakage efficient anti-glitch filters with variable delay stages.

BACKGROUND

It is sometimes advantageous to delay propagation of a signal through an integrated circuit. A common approach to create such signal delays is to insert a delay circuit, or delay “line,” into a signal path. One well known class of delay circuits is a variable delay circuit. For example, a variable delay circuit can function by selectably varying a number of delay elements to produce different amounts of delay to a signal propagating through such elements. However, in general, variable delay lines can generate “glitches” or deleterious spurious signals. For example, changing a delay duration while a transition is propagating through the delay line can sometimes cause the transition to arrive twice at the output, producing a glitch. Further, it is generally desirable for delay circuit designs to be efficient in terms of integrated circuit die area, active power consumption and static power (leakage current) consumption.

SUMMARY OF THE INVENTION

A leakage efficient anti-glitch filter with variable delay stages is disclosed. In accordance with a first embodiment of the present invention, a leakage efficient anti-glitch filter with variable delay stages comprises a plurality of variable delay stages and a coincidence detector element for detecting coincidence of an input signal to the delay element and an output of the delay element. The plurality of variable delay stages may comprise stacked inverter circuits or stacked NAND circuits.

In accordance with another embodiment of the present invention, a method of delaying an electronic signal is disclosed. An electronic signal is accessed. A delayed version of the electronic signal is produced through a variable number of delay stages. The electronic signal and the delayed version of the electronic signal are combined to produce a glitch-free delayed electronic signal. The glitch-free delayed electronic signal transitions in response to a coincidence of the electronic signal and the delayed version of the electronic signal.

In accordance with still another embodiment of the present invention, an electronic circuit is disclosed. The electronic circuit comprises a delay line comprising a plurality of stacked inverters coupled in series. Each of the plurality of stacked inverters comprises at least two devices of a first type coupled in series, coupled in series to at least two devices of an opposite type coupled in series. The electronic circuit further comprises a plurality of multiplexers corresponding to the plurality of delay stages for selecting between a signal present at a desired delay stage and a signal propagating from beyond a delay stage corresponding to a multiplexer. The electronic circuit further comprises a coincidence detector circuit. The coincidence detector circuit comprises first and second devices of a first type coupled in series, coupled in series to third and fourth devices of a second type coupled in series. An input to the delay line is coupled to gates of the first and fourth devices and an output of the delay line is coupled to gates of the second and third devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of a novel leakage efficient anti-glitch filter, in accordance with embodiments of the present invention.

FIG. 2 is a timing diagram illustrating a function of a leakage efficient anti-glitch filter, in accordance with embodiments of the present invention.

FIG. 3 illustrates a flow chart describing a method of filtering glitches from an electronic signal, in accordance with embodiments of the present invention.

FIG. 4 illustrates a leakage efficient anti-glitch filter with variable delay stages, in accordance with embodiments of the present invention.

FIG. 5 illustrates a flow chart describing a method of delaying an electronic signal, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the present invention, leakage efficient anti-glitch filter with variable delay stages, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one skilled in the art that the present invention may be practiced without these specific details or with equivalents thereof. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.

Leakage Efficient Anti-Glitch Filter

Embodiments in accordance with the present invention are described in the context of design and operation of integrated semiconductors. More particularly, embodiments of the present invention relate to leakage efficient anti-glitch filters. It is appreciated, however, that elements of the present invention may be utilized in other areas of semiconductor design and operation.

The following description of embodiments in accordance with the present invention is directed toward pFETs (or p-type metal oxide semiconductor field effect transistors (MOSFETS)) formed in surface N-wells and/or nFETs (or n-type MOSFETS) formed in surface P-wells when a p-type substrate and an N-well process are utilized. It is to be appreciated, however, that embodiments in accordance with the present invention are equally applicable to nFETs (or n-type MOSFETS) formed in surface P-wells and/or pFETs (or p-type MOSFETS) formed in surface N-wells when an n-type substrate and a P-well process are utilized. Consequently, embodiments in accordance with the present invention are well suited to semiconductors formed in both p-type and n-type materials, and such embodiments are considered within the scope of the present invention.

FIG. 1 illustrates a schematic of a novel leakage efficient anti-glitch filter 100, in accordance with embodiments of the present invention. Leakage efficient anti-glitch filter 100 comprises an optional input buffer 110, a leakage efficient delay line 120 and a coincidence detector element 130.

Optional input buffer 110 can provide transition speed isolation. For example, without optional input buffer 110, leakage efficient delay line 120 presents six loads at its input. Such loading may represent an undesirable loading condition to the source of a signal. In accordance with embodiments of the present invention, input buffer 110 is well suited to a variety of buffering structures, including, for example, conventional or stacked inverters and non-inverting buffers.

It is to be appreciated that static power consumption in modern semiconductor processes, e.g., processes with a minimum feature size of about 0.13 microns and smaller, is no longer a negligible component of total power consumption. For such processes, static power may be one-half of total power consumption. Further, static power, as a percentage of total power, is tending to increase with successive generations of semiconductor process.

Embodiments in accordance with the present invention offer significant advantages in reducing leakage current, or static power consumption, in comparison with the conventional art. In general, an inverter stage, whether conventional or stacked, forms a leakage path, e.g., a series “string” of devices coupled from operating voltage (Vdd) to ground. As current leaks out of such leakage paths, static power is consumed by the inverter stage. As described more completely in U.S. patent application Ser. No. 10/864,271, entitled “Stacked Inverter Delay Chain” to Masleid and Burr, incorporated herein by reference in its entirety, a delay chain comprising stacked inverters can require fewer stages than a delay chain comprising conventional inverters to produce a comparable delay. For example, each stage of stacked inverters will generally produce greater delay than a conventional, non-stacked inverter. Fewer stages result in fewer leakage paths, resulting in advantageously less leakage current, resulting in less static power consumption.

Further, such leakage paths within a stacked inverter suffer less leakage than a conventional inverter, yielding additional beneficial leakage reductions. In a conventional inverter, exactly one transistor is on while the other transistor is off. As an unfortunate consequence, approximately the full bias voltage is applied to the off transistor, resulting in a maximum possible leakage for the off transistor.

In contrast, in a stacked inverter multiple transistors are either on or off in series. For example, in the embodiment of FIG. 1, for a “high” state, two transistors are on, while two transistors are off. Consequently, each “off” transistor has significantly less than full bias voltage applied. It is appreciated that leakage current generally decreases exponentially as voltage decreases. For example, a factor of two reduction in off bias voltage produces about a factor of eight reduction in leakage current per leakage path.

It is to be further appreciated that such leakage induces non zero voltages at intermediate nodes between the off transistors. Such voltages induce body effects in the transistors. Such body effects increase the threshold voltage of the affected transistors. An increased threshold voltage generally produces beneficial decreases in leakage current.

Consequently, in addition to a decrease in a number of leakage paths, in accordance with embodiments of the present invention, the leakage current of each path is very beneficially reduced due to an induced body effect and a highly non-linear relationship between bias voltage and leakage current. Thus, leakage efficient delay line 120 significantly reduces static power consumption, in comparison to a delay line comprising conventional inverters.

Leakage efficient delay line 120 comprises a plurality of stacked inverters. The output of a first stacked inverter is generally coupled to the input of a subsequent stacked inverter. It is to be appreciated that more or fewer stacked inverters can be included in leakage efficient delay line 120 in order to achieve differing signal delay values, in accordance with embodiments of the present invention.

For example, physical differences between electrons and holes, and between n-type and p-type dopants, as well as constructive differences in device geometry and dopant placement, result in differences in efficiency between n-type devices and p-type devices. Because electron mobility is higher than hole mobility, n-type devices are more efficient than p-type devices. However, the degree of difference depends on constructive differences that can vary with process. Such physical and constructive differences also produce other behavior differences, such as a difference in sensitivity to body effects. Consequently, different levels of benefit, e.g., in leakage reduction, are to be expected between stacks of n-type devices and stacks of p-type devices. To allow for such effects, in accordance with embodiments of the present invention, it is possible to stack different numbers of transistors on either or both legs of a stacked inverter. Such variations allow increases in load and/or decreases in drive capability, enabling a wide variety of delay values, as well as enabling differing body effects.

In contrast to a conventional inverter, the stacked inverters of leakage efficient delay line 120 comprise more than a single p-type device coupled to a single n-type device. Rather, the stacked inverters of leakage efficient delay line 120 comprise multiple p-type devices and multiple n-type devices. In the embodiment of FIG. 1, the inverters are shown as comprising two p-type devices and two n-type devices. It is appreciated that embodiments of the present invention are well suited to a variety of stacking combinations of p-type and n-type devices, as well as to other delay elements, for example leakage efficient, e.g., stacked, NAND gates.

As will be discussed further below, the signal delay produced by leakage efficient delay line 120 is related to the duration of spurious signals, or glitches, that are to be filtered by leakage efficient anti-glitch filter 100.

Still referring to FIG. 1, coincidence detector element 130 functions to combine signal A 111, the input of leakage efficient delay line 120, with signal B 121, the output of leakage efficient delay line 120. Coincidence detector element 130 will only propagate a signal when signals A 111 and B 121 coincide.

In the embodiment of FIG. 1, coincidence detector element 130 comprises a “Mueller-C” element. It is appreciated that embodiments of the present invention are well suited to other types of coincidence detectors. A Mueller-C element is a well known special type of NAND gate characterized by propagating a state change only when both of its inputs agree. For example, a true NAND (or AND) gate can transition from true to false when only one of its inputs changes from true to false. In contrast, coincidence detector element 130 does not transition until all inputs agree, e.g., both inputs change from true to false.

For example, only when both signal A 111 and signal B 121 are low will the pFET devices of coincidence detector element 130 turn on and pull output signal C 131 to a high condition. Similarly, only when both signal A 111 and signal B 121 are high will the nFET devices of coincidence detector element 130 turn on and pull output C 131 to a low condition.

Conventionally, coincidence detector elements, e.g., a Mueller-C element, are generally utilized to compare or combine disparate signals. For example, a conventional use of such elements is to determine completion of a logic function.

In contrast, in accordance with embodiments of the present invention, coincidence detector element 130 receives as input the input to leakage efficient delay line 120 and the output of leakage efficient delay line 120. Thus, coincidence detector element 130 receives a signal and a delayed version of the same signal.

Leakage efficient anti-glitch filter 100 suppresses any glitch or spurious signal characterized as having a duration of less than the delay characteristic of leakage efficient delay line 120.

It is generally desirable for a delay circuit, e.g., leakage efficient delay line 120, to track speed changes of other circuitry of an integrated circuit. It is appreciated that a variety of factors, e.g., operating voltage, operating temperature and/or manufacturing process variations, can affect the speed of operation of an integrated circuit. For example, if other circuits of an integrated circuit operate faster, generally less absolute delay is required from a delay circuit for the overall circuit to function. By way of further example, if other circuits of an integrated circuit operate more slowly, it may be advantageous for leakage efficient delay line 120 to operate more slowly, e.g., with greater absolute delay, in order to remove longer glitches generated by such slower operating other circuitry. Because embodiments in accordance with the present invention comprise stacked devices, they are similar to many logic circuits that also comprise stacked devices, e.g., NAND and/or NOR logic gates. Consequently, embodiments in accordance with the present invention match or track changes in operating speed of complex logic more accurately than delay chains comprising very simple inverters.

Embodiments in accordance with the present invention are thus shown to offer significant and highly beneficial improvements in tracking timing changes of other circuits and in static power (leakage current) consumption in comparison to the conventional art.

FIG. 2 is a timing diagram 200 illustrating a function of leakage efficient anti-glitch filter 100, in accordance with embodiments of the present invention. Timing diagram 200 shows a relationship among exemplary signals associated with leakage efficient anti-glitch filter 100.

Signal A 210 is an input waveform to leakage efficient anti-glitch filter 100. Signal A 210 is one possible signal corresponding to signal A 111 of FIG. 1. Signal A 210 comprises a glitch 220 and a desired transition 230.

Signal B 240 is a delayed version of signal A 210, corresponding to the output B 121 of leakage efficient delay line 120 (FIG. 1). Signal B 240 is a reproduction of signal A 210, delayed by a duration 245. It is to be appreciated that duration 245 corresponds to the delay of leakage efficient delay line 120 of FIG. 1. Signal B 240 comprises a glitch 250, corresponding to glitch 220, and a desired transition 260, corresponding to desired transition 230. It is to be appreciated that glitch 250 and desired transition 260 are delayed by duration 245 in relation to their corresponding elements of signal A 210. It is to be further appreciated that glitch 220 is of a time duration that is less than delay duration 245.

Signal C 270 is an output of leakage efficient delay line 120 as viewed at output C 131 (FIG. 1) when excited by signal A 210. It is appreciated that desired transition 280 of signal C 270 does not occur until both signal A 210 and signal B 240 agree. For example, at a time before desired transition 280, signal B 240 is low. Consequently, changes in signal A 210, e.g., glitch 220 and/or desired transition 230, do not affect signal C 270.

At the time of desired transition 280, signal A 210 is high, and signal B 240 transitions to high. Consequently, signal C 270 transitions to low, producing desired transition 280. As signal A 210 is high for the remaining duration of timing diagram 200, changes in signal B 240, e.g., glitch 250 and/or desired transition 260, do not affect signal C 270.

Thus, in accordance with embodiments of the present invention, a spurious signal, e.g., glitch 220, present on an input to leakage efficient anti-glitch filter 100 (FIG. 1) is removed, and is not present on the output of leakage efficient anti-glitch filter 100.

It is to be appreciated that signal C 270 is inverted relative to signal A 210. Referring once again to FIG. 1, the output of leakage efficient anti-glitch filter 100, e.g., signal C 131, can be inverted by an inverting circuit (not shown) to obtain a positive phase relationship to a signal input to leakage efficient anti-glitch filter 100. Alternatively, an inverting optional input buffer 110 can “pre-invert” a signal input to leakage efficient anti-glitch filter 100 in order to obtain a positive phase relationship between an input signal and an output signal.

FIG. 3 illustrates a flow chart describing a method 300 of filtering glitches from an electronic signal, in accordance with embodiments of the present invention. In 310, an electronic signal is accessed. For example, referring once again to FIG. 1, signal A 111 is accessed by leakage efficient anti-glitch filter 100.

In 320, a delayed version of the electronic signal is produced. For example, signal B 121 is a delayed version of signal A 111 in FIG. 1. In 330, the electronic signal and the delayed version of the electronic signal are combined to produce a glitch-free electronic signal. The glitch-free electronic signal transitions in response to a coincidence of the electronic signal and the delayed version of the electronic signal.

Leakage Efficient Anti-Glitch Filter with Variable Delay Stages

FIG. 4 illustrates a leakage efficient anti-glitch filter with variable delay stages 400, in accordance with embodiments of the present invention. It is to be appreciated that the embodiment of FIG. 4 illustrates a variable delay chain with uniform delay stages. Embodiments in accordance with the present invention are well suited to a wide variety of delay chains, including non-uniform, e.g., tapered, delay chains and/or delay chains with variable elements per tap. It is to be further appreciated that embodiments in accordance with the present invention are well suited to greater numbers of selectable delay stages than illustrated.

Leakage efficient anti-glitch filter with variable delay stages 400 comprises a plurality of selectable delay stages, e.g., selectable delay stages 440 and 450. It is appreciated that selectable delay stages 440 and 450 form a variable delay line. Each selectable delay stage comprises a delay element, e.g., delay element 441 of selectable delay stage 440.

In the embodiment of FIG. 4, delay element 441 comprises a stacked 2-input NAND gate. It is to be appreciated that other types of delay elements, e.g., a stacked inverter, are well suited to embodiments in accordance with the present invention. It is to be appreciated that a delay chain comprising stacked NAND gates can require fewer stages than a delay chain comprising conventional NAND gates to produce a comparable delay. For example, each stage of stacked NAND gates will generally produce greater delay than a conventional, non-stacked NAND gate. Fewer stages result in fewer leakage paths, resulting in advantageously less leakage current, resulting in less static power consumption.

It is appreciated that leakage current generally decreases exponentially as voltage decreases. For example, a factor of two reduction in off bias voltage produces about a factor of eight reduction in leakage current per leakage path. Consequently, as the operating voltage is distributed across more devices in a stacked NAND gate, as opposed to a conventional NAND gate, each “off” transistor has significantly less than full bias voltage applied, and that this effect further reduces leakage current.

It is to be further appreciated that current leakage induces non zero voltages at intermediate nodes between the off transistors. Such voltages induce body effects in the transistors. Such body effects increase the threshold voltage of the affected transistors. An increased threshold voltage generally produces beneficial decreases in leakage current.

Consequently, in addition to a decrease in a number of leakage paths, in accordance with embodiments of the present invention, the leakage current of each path is very beneficially reduced due to an induced body effect and a highly non-linear relationship between bias voltage and leakage current. Thus, delay element 441 significantly reduces static power consumption, in comparison to a delay element comprising conventional NAND gates or conventional inverters.

The 2-input NAND gate of the illustrated embodiment advantageously tracks timing changes, e.g., due to process variation, operating voltage and/or operation temperature, of other similarly constructed elements. For example, co-pending, commonly owned U.S. patent application Ser. No. 11/021,221, filed Dec. 23, 2004, entitled “A Configurable Delay Chain with Switching Control for Tail Delay Elements” to Masleid, incorporated herein in its entirety by reference, discloses a variable delay chain comprising a plurality of 2-input NAND gates as delay and control elements. Delay element 441 can beneficially track timing changes of such exemplary variable delay chain elements.

Leakage efficient anti-glitch filter with variable delay stages 400 further comprises a multiplexer element, e.g., multiplexer element 442 of selectable delay stage 440. In general, the multiplexer element of a selectable delay stage selects between a signal present at the output of the associated delay stage delay element and a signal propagated from subsequent delay stages.

Multiplexer element 442 selects between a signal propagated from subsequent delay stages, e.g., signal 451 propagated from the multiplexer element of selectable delay stage 450, and signal 452 associated with its own selectable delay stage 440. A bit value in latch 455 determines which signal, 451 or 452, is selected by multiplexer element 442. A more detailed description of the operation of multiplexer element 442 is presented in co-pending, commonly owned U.S. patent application Ser. No. 11/021,632, filed Dec. 23, 2004, entitled “Power Efficient Multiplexer” to Masleid, incorporated herein by reference in its entirety.

For example, if multiplexer element 442 selects the output of another multiplexer element, e.g., signal 451, more delay elements have delayed an original signal, and the total delay is longer. In contrast, if multiplexer element 442 selects the output of its own delay element, e.g., signal 452, fewer delay elements have delayed an original signal, and the total delay is lessened.

In general, variable delay lines, for example, the variable delay line formed by selectable delay stages 440 and 450, can generate “glitches” or deleterious spurious signals. For example, changing a multiplexer control signal while a transition is propagating through the delay line can sometimes cause the transition to arrive twice at the output, producing a glitch.

Coincidence detector element 430 generally corresponds to coincidence detector element 130 of FIG. 1. Referring once again to FIG. 4, it is to be appreciated that the variable delay line formed by selectable delay stages 440 and 450 produces a delayed version of input signal A 411. That delayed version of input signal A 411 is signal B 421. As described previously, coincidence detector element 430 does not change state until input signal A 411 and signal B 421 coincide, or are at the same level.

Advantageously, any glitch produced by incrementing the delay of a separate variable delay line formed by selectable delay stages similar to stages 440 and 450 will generally be of shorter duration than the delay produced by delay stages 440 and 450, when the appropriate latches are set to one. As coincidence detector element 430 serves to filter glitches of duration less than the delay between its inputs, e.g., the delay between signal A 411 and signal B 421, coincidence detector element 430 will generally filter any glitch shorter than the total delay of delay stages 440 and 450.

For example, if a separate variable delay line is incremented two stages at a time, then the glitch filter should contain a delay of four stages to allow for a 2:1 process mismatch.

It is to be appreciated that this general timing relationship will hold for a plurality of delay stages, in accordance with embodiments of the present invention.

Optional input buffer 410 can provide transition speed isolation. For example, without optional input buffer 410, leakage efficient anti-glitch filter with variable delay stages 400 presents six loads at its input. Such loading may represent an undesirable loading condition to the source of a signal. In accordance with embodiments of the present invention, input buffer 410 is well suited to a variety of buffering structures, including, for example, conventional or stacked inverters and non-inverting buffers.

A timing diagram for signals A 441, B 421 and C 431 of FIG. 4 corresponds to timing diagram 200 of FIG. 2. Signal B 421 is a delayed version of signal A 441. It is appreciated that delay is a primary function of selectable delay stages 440 and 450. It is to be further appreciated that, in general, a delay produced by selectable delay stages 440 and/or 450 will comprise a different duration than a delay produced by leakage efficient delay line 120 of FIG. 1.

As described previously, coincidence detector element 430 does not change state until input signal A 411 and signal B 421, a delayed version of signal A 411, coincide, or are at the same level. It is appreciated that a desired transition of signal C 431, corresponding to a desired transition of signal A 411, does not occur until both signal A 411 and signal B 421 agree.

Thus, in accordance with embodiments of the present invention, a spurious signal, e.g., a glitch applied to the variable delay line formed by selectable delay stages 440 and/or 450 is removed and is not present on the output signal C 431 of leakage efficient anti-glitch filter with variable delay stages 400 if the glitch present on the input of selectable delay stage 440 is of less duration than the delay produced by selectable delay stages 440 and/or 450.

It is to be appreciated that signal C 431 is inverted relative to signal A 411. The output of leakage efficient anti-glitch filter with variable delay stages, e.g., signal C 431, can be inverted by an inverting circuit (not shown) to obtain a positive phase relationship to a signal input to leakage efficient anti-glitch filter 100. Alternatively, an inverting optional input buffer 410 can “pre-invert” a signal input to leakage efficient anti-glitch filter with variable delay stages 400 in order to obtain a positive phase relationship between an input signal and an output signal.

FIG. 5 illustrates a flow chart describing a method 500 of delaying an electronic signal, in accordance with embodiments of the present invention. In 510, an electronic signal is accessed. For example, referring once again to FIG. 4, signal A 411 is accessed by selectable delay stage 440.

In 520, a delayed version of the electronic signal is produced through a variable number of delay stages. For example, signal B 421 is a delayed version of signal A 411 in FIG. 4. Signal B 421 can be delayed through a variable number of delay stages, e.g., selectable delay stages 440 and/or 450.

In 530, the electronic signal and the delayed version of the electronic signal are combined to produce a glitch-free delayed electronic signal. The glitch-free delayed electronic signal transitions in response to a coincidence of the electronic signal and the delayed version of the electronic signal.

Embodiments in accordance with the present invention thus provide a leakage efficient anti-glitch filter with variable delay stages comprising desirable static power (leakage current) consumption in comparison to the conventional art.

Embodiments in accordance with the present invention, leakage efficient anti-glitch filter with variable delay stages, are thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims. 

1. An anti-glitch filter circuit comprising: a plurality of variable delay stages; and a coincidence detector element for detecting coincidence of an input signal to said plurality of variable delay stages and an output signal of said plurality of variable delay stages; and a glitch-filtered output signal output from said coincidence detector element.
 2. The anti-glitch filter circuit of claim 1 wherein in said glitch-filtered output signal, spurious signals characterized as having a duration of less than a delay of said plurality of variable delay stages are substantially eliminated.
 3. The anti-glitch filter circuit of claim 2 wherein said coincidence detector comprises a Mueller-C element.
 4. The anti-glitch filter circuit of claim 2 wherein said plurality of variable delay stages comprises a plurality of inverters.
 5. The anti-glitch filter circuit of claim 4 wherein said plurality of inverters comprises a stacked inverter circuit.
 6. The anti-glitch filter circuit of claim 5 wherein said stacked inverter circuit comprises at least two p-type devices coupled in series with at least two n-type devices.
 7. The anti-glitch filter circuit of claim 6 wherein said stacked inverter circuit is characterized in having a greater switching time duration than an inversion performed by an inverter comprising only two active devices.
 8. The anti-glitch filter circuit of claim 6 wherein said stacked inverter circuit is characterized in having less leakage current than an inverter comprising only two active devices.
 9. The anti-glitch filter circuit of claim 6 wherein said stacked inverter circuit comprises: a first plurality of devices of a first type coupled in series; a second plurality of devices of a second type coupled in series, wherein said second type is opposite to said first type; and wherein said first and said second pluralities of devices are coupled in series and comprise at least five active devices.
 10. The anti-glitch filter circuit of claim 2 wherein said plurality of variable delay stages comprises a plurality of NAND gates.
 11. The anti-glitch filter circuit of claim 10 wherein said plurality of NAND gates comprises a stacked NAND gate.
 12. The anti-glitch filter circuit of claim 11 wherein said stacked NAND gate comprises two p-type devices coupled in series with four n-type devices.
 13. The anti-glitch filter circuit of claim 12 wherein said stacked NAND gate is characterized in having a greater switching time duration than a switching time duration of a NAND gate comprising only four active devices.
 14. The anti-glitch filter circuit of claim 12 wherein said stacked NAND gate is characterized in having less leakage current than a NAND gate comprising only four active devices.
 15. The anti-glitch filter circuit of claim 12 wherein said stacked NAND gate comprises: a first plurality of devices of a first type coupled in series; a second plurality of devices of second type coupled in series, wherein said second type is opposite to said first type; a third plurality of devices of said second type coupled in series; a fourth plurality of devices of said first type coupled in series; wherein said first second and third pluralities of devices are coupled in series; wherein gates of said first and third pluralities of devices are coupled to a first input signal; wherein gates of said second and fourth pluralities of devices are coupled to a second input signal; and wherein non-power terminals of said first and fourth pluralities of devices are coupled to form an output signal of said stacked NAND gate.
 16. A method of delaying an electronic signal comprising: accessing an electronic signal; producing a delayed version of said electronic signal through a variable number of delay stages; and combining said electronic signal and said delayed version of said electronic signal to produce a substantially glitch-free delayed electronic signal, wherein said glitch-free delayed electronic signal transitions in response to a coincidence of said electronic signal and said delayed version of said electronic signal.
 17. The method of claim 16 wherein said combining comprises filtering glitches characterized as having a duration of less than a delay of said delayed version of said electronic signal.
 18. The method of claim 16 wherein said combining is performed using a Mueller-C element.
 19. The method of claim 16 wherein said producing is performed using a plurality of inverters.
 20. The method of claim 19 wherein said plurality of inverters comprise a stacked inverter circuit.
 21. The method of claim 20 wherein said stacked inverter circuit comprises at least two p-type devices coupled in series with at least two n-type devices.
 22. The method of claim 21 wherein said stacked inverter circuit is characterized in having a greater switching time duration than an inversion performed by an inverter comprising only two active devices.
 23. The method of claim 21 wherein said stacked inverter circuit is characterized in having less leakage current than an inverter comprising only two active devices.
 24. The method of claim 21 wherein said stacked inverter circuit comprises: a first plurality of devices of a first type coupled in series; a second plurality of devices of a second type coupled in series, wherein said second type is opposite to said first type; and wherein said first and said second pluralities of devices are coupled in series and comprise at least five active devices.
 25. An electronic circuit comprising: a plurality of delay stages comprising a plurality of stacked inverters coupled in series, each of said plurality of stacked inverters comprising: at least two devices of a first type coupled in series, coupled in series to at least two devices of a second type coupled in series, wherein said second type is opposite to said first type; a plurality of multiplexers corresponding to said plurality of delay stages for selecting between a signal present at a desired delay stage and a signal propagating from beyond a delay stage corresponding to a multiplexer; a coincidence detector circuit comprising: first and second devices of a first type coupled in series, coupled in series to third and fourth devices of a second type coupled in series; and wherein an input to said plurality of delay stages is coupled to gates of said first and fourth devices and an output of said plurality of delay stages is coupled to gates of said second and third devices.
 26. The electronic circuit of claim 25 wherein gates of each device comprising each of said plurality of stacked inverters are coupled together.
 27. The electronic circuit of claim 25 wherein each of said plurality of stacked inverters are substantially identical.
 28. The electronic circuit of claim 25 wherein each of said plurality of stacked inverters comprises two p-type devices.
 29. The electronic circuit of claim 25 wherein each of said plurality of stacked inverters comprises three n-type devices.
 30. The electronic circuit of claim 25 further comprising an inverter coupled to said input of said plurality of delay stages. 